*Optical* *interference *is just the interaction of two or more light waves. Optical interference is useful in many applications, so you need to understand some basic equations related to this optical phenomenon. The following equations allow you to calculate various quantities related to optical interference in the two most common interference arrangements.

**The location of the bright and dark fringes in Young's two-slit interference arrangement:**The following equations allow you to calculate the location of the*bright fringes*(where constructive interference occurs) and*dark fringes*(where destructive interference occurs):**The phase shift due to the film thickness in thin film interference:**When light is incident straight onto a thin film (such as an oil slick on the surface of a pool of water), light rays reflecting from the top and the bottom of the film interfere (either constructively or destructively depending on the film thickness and the wavelength of the light). The following equations determine constructive or destructive interference depending on whether the phase shift produced by reflection needs to be shifted by half the wavelength (the first equation) or maintained (the second equation):